The activation of cell membrane receptors by transmitters leads to the activation of the second messenger system. Adenylate cyclase synthesizes active cyclic AMP (cAMP) or cyclic GMP (cGMP) from AMP and GMP. These lead, for example, to relaxation in smooth muscle cells or to inhibition of mediator release or synthesis in inflammatory cells. The breakdown of the second messenger cAMP and cGMP is carried out by the phosphodiesterases (PDE). To date, 7 families of PDE enzymes (PDE1-7) are known, which differ by their substrate specificity (cAMP, cGMP or both) and the dependence on other substrates (e.g. calmodulin). These isoenzymes have different functions in the body and are prominent to different extents in the individual cell types (Beave J A, Conti M and Heaslip R J, Multiple cyclic nucleotide phosphodiesterases, Mol. Pharmacol. 1994, 46: 399–405; Hall I P, Isoenzyme selective phosphodiesterase inhibitors; potential clinical uses, Br. J. clin. Pharmacol. 1993, 35: 1–7). As a result of inhibition of the various PDE isoerzyme types, there is an accumulation of cAMP or cGMP in the cells, which can be therapeutically utilized (Torphy T J, Livi G P, Christensen S B, Novel Phosphodiesterase Inhibitors for the Therapy of Asthma, Drug News and Perspectives 1993, 6: 203–214).
In the cells important for allergic inflammation (lymphocytes, mast cells, eosinophilic granulocytes, macrophages), the prevailing PDE isoenzyme is of type 4 (Torphy, J T. and Undem, B. J., Phosphodiesterase inhibitors: new opportunities for the treatment of asthma, Thorax 1991, 46: 512–523). The inhibition of PDE 4 by suitable inhibitors is therefore considered as an important starting point for the therapy of a large number of allergically induced disorders (Schudt Ch, Dent G, Rabe K, Phosphodiesterase Inhibitors, Academic Press London 1996).
An important property of phosphodiesterase 4 inhibitors is the inhibition of the release of tumour necrosis factor α (TNFα) from inflammatory cells. TNFα is an important pro-inflammatory cytokine, which affects a large number of biological processes. TNFα is released, for example, from activated macrophages, activated T lymphocytes, mast cells, basophils, fibroblasts, endothelial cells and astrocytes in the brain. It has a self-activating effect on neutrophils, eosinophils, fibroblasts and endothelial cells, as a result of which various tissue-destroying mediators are released. In monocytes, macrophages and T lymphocytes, TNFα brings about the increased production of further pro-inflammatory cytokines such as GM-CSF (granulocyte-macrophage colony-stimulating factor) or interleukin-8. TNFα plays a central part due to its inflammation-promoting and catabolic action in a large number of disorders, such as inflammation of the airways, inflammation of the joints, endotoxic shock, tissue rejection, AIDS and numerous other immunological disorders. Inhibitors of phosphodiesterase 4 are thus also suitable for the therapy of disorders of this type which are associated with TNFα.
Chronic obstructive pulmonary diseases (COPD) are widespread in the population and also have great economic importance. Thus COPD diseases cause about 10–15% of all illness costs in the developed countries and about 25% of all cases of death in the USA are to be attributed to this cause (Norman P.: COPD: New developments and therapeutic opportunities, Drug News Perspect. 11 (7), 431–437, 1998), however the patients at the time of death are usually over 55 years old (Nolte D.: Chronische Bronchitis—eine Volkskrankheit multifaktorieller Genese. Atemw.-Lungenkrkh. [Chronic bronchitis—a widespread disease of multifactorial origin]. 20 (5), 260–267, 1994). The WHO estimates that COPD will be the third most frequent cause of death within the next 20 years.
The syndrome of chronic obstructive lung diseases (COPD) summarizes various syndromes of chronic bronchitis with the symptoms coughing and expectoration and progressive and irreversible impairment of lung function (exhalation is particularly affected). The course of the disease is episodic and often complicated by bacterial infections (Rennard S. I.: COPD: Overview of definitions, Epidemiology, and factors influencing its development. Chest, 113 (4) Suppl., 235S–241S, 1998). In the course of the disease, the lung function continuously decreases, the lungs become increasingly emphysematous and the respiratory distress of the patients is obvious. This disease clearly adversely affects the quality of life of the patients (dyspnoea, low exercise tolerance) and significantly reduces their life expectancy. The main risk factor besides environmental factors is smoking (Kummer F.: Asthma und COPD. Atemw.-Lungenkrkh. 20 (5), 299–302, 1994; Rennard S. I.: COPD: Overview of definitions, Epidemiology, and factors influencing its development. Chest, 113 (4) Suppl., 235S–241S, 1998) and therefore men are clearly more often affected than women. As a result of the change in living habits and the increase in the number of smokers, this picture, however, will change in future.
The current therapy aims only at the alleviation of the symptoms, without causally intervening in the progression of the disease. The use of long-acting Beta2 agonists (e.g. salmeterol) possibly in combination with muscarinergic antagonists (e.g. ipratropium) improves the lung function by bronchodilatation and is employed routinely (Norman P.: COPD: New developments and therapeutic opportunities, Drugs News Perspect. 11 (7), 431–437, 1998). A large part in the COPD episodes is played by bacterial infections, which have to be treated with antibiotics (Wilson R.: The role of infection in COPD, Chest, 113 (4) Suppl., 242S–248S, 1998; Grossman R. F.: The value of antibiotics and the outcomes of antibiotic therapy in exacerbations of COPD. Chest, 113 (4) Suppl., 249S–255S, 1998). The therapy of this disease is unsatisfactory as yet, particularly with respect to the continuous decrease in lung function. New therapeutic approaches which affect inflammatory mediators, proteases or adhesion molecules could be very promising (Barnes P. J.: Chronic obstructive disease: new opportunities for drug development, TiPS 10 (19), 415–423, 1998).
Independently of the bacterial infections complicating the disease, a chronic inflammation which is dominated by neutrophilic granulocytes is found in the bronchi. The mediators and enzymes released by neutrophilic granulocytes, inter alia, have been held responsible for the structural changes observed in the airways (emphysema). The inhibition of the activity of the neutrophilic granulocytes is thus a rational approach to prevent or to slow down progression of COPD (impairment of lung function parameters). An important stimulus for the activation of the granulocytes is the pro-inflammatory cytokine TNFα (tumour necrosis factor). Thus it is known that TNFα stimulates the formation of oxygen radicals by neutrophilic granulocytes (Jersmann, H. P. A.; Rathjen, D. A. and Ferrante A.: Enhancement of LPS-induced neutrophil oxygen radical production by TNFα, Infection and Immunity, 4, 1744–1747, 1998). PDE4 inhibitors can very effectively inhibit the release of TNFα from a large number of cells and thus suppress the activity of the neutrophilic granulocytes. The non-specific PDE inhibitor pentoxifylline is able to inhibit both the formation of oxygen radicals and the phagocytosability of neutrophilic granulocytes (Wenisch, C.; Zedtwitz-Liebenstein, K.; Parschalk, B. and Graninger W.: Effect of pentoxifylline in vitro on neutrophil reactive oxygen production and phagocytic ability assessed by flow cytometry, Clin. Drug. Invest., 13(2):99–104, 1997).
Various PDE 4 inhibitors are already known. As a matter of priority, these are xanthine derivatives, rolipram analogues or nitraquazone derivatives (general survey in: Karlsson J-A, Aldos D, Phosphodiesterase 4 inhibitors for the treatment of asthma, Exp. Opin. Ther. Patents 1997, 7: 989–1003). Until now, it was not possible to use any of these compounds clinically. It had to be established that the known PDE 4 inhibitors also have various side-effects such as nausea and emesis, which it was not possible to suppress adequately until now. The discovery of new PDE 4 inhibitors with better therapeutic breadth is therefore necessary.
Although indoles have been playing an important part for many years in the development of new active compounds for various indications, until now hydroxyindoles were completely unknown as inhibitors of PDE 4.